The present invention relates to variable valve timing apparatuses that are employed in engines. More particularly, the present invention relates to a variable timing apparatus that includes a phase adjustor and a lift adjustor for controlling valve timing with a three-dimensional cam.
Engine variable valve timing apparatuses control the valve timing of intake valves and exhaust valves in accordance with the operating state of the engine. A variable valve timing apparatus generally includes a timing pulley and a sprocket, which synchronously rotates a camshaft with a crankshaft.
Japanese Unexamined Patent Publication No. 9-60508 describes a typical variable timing apparatus. As shown in FIGS. 10, 11, and 12, the variable valve timing apparatus includes a phase adjustor arranged on one end of a camshaft 202. FIG. 10 is a cross-sectional view taken along line 10--10 in FIG. 11, while FIG. 11 is a cross-sectional view taken along line 11--11 in FIG. 10. FIG. 12 is a cross-sectional view taken along line 12--12 in FIG. 11.
A sprocket 204, which is driven by a crankshaft (not shown), is coupled with a housing 206 and supported to rotate integrally with the housing 206. A vane rotor 208 is arranged in the center of the housing 206 and secured to the end of the camshaft 202 to rotate integrally with the camshaft 202.
Vanes 210 project outward from the hub of the vane rotor 208 to contact the inner wall of the housing 206. Partititions 212 project inward from the housing 206 to contact the hub surface of the vane rotor 208. Cavities 214 are defined between the partitions 212. A first pressure chamber 216 and a second pressure chamber 218 are defined in each cavity 214 between each vane 210 and the partitions 212.
Hydraulic pressure is communicated to the first and second pressure chambers 216, 218 to rotate the vane rotor 208 relative to the housing 206. As a result, the rotational phase of the vane rotor 208 relative to the housing 206 is adjusted. This, in turn, adjusts the rotational phase of the camshaft 202 relative to the crankshaft.
The camshaft 202 has a journal 224, which is supported by a bearing 222 formed in a cylinder head of the engine. A first oil channel, which is connected with a hydraulic unit 220, extends through the cylinder head and connects to an oil groove 226 extending along the peripheral surface of the journal 224. The oil groove 226 is connected to oil conduits 227, 228, which extend through the camshaft 202. The oil conduit 228 is further connected to oil conduits 230, 232, which extend through the vane rotor 208 and lead into the first pressure chambers 216. Accordingly, hydraulic pressure is communicated between the hydraulic unit 220 and the first pressure chambers 216 through the first oil channel, the oil groove 226 and the oil conduits 227, 228, 230, 232.
A second oil channel, which is connected with the hydraulic unit 220, extends through the cylinder head and connects to an oil groove 236 extending along peripheral surface of the journal 224. The oil groove 236 is connected to an oil conduit 238, which extends through the camshaft 202. The oil conduit 238 is further connected to oil conduits 240, 242, 244, which extend through the vane rotor 208 and lead into the second pressure chambers 218. Accordingly, hydraulic pressure is communicated between the hydraulic unit 220 and the second pressure chambers 218 through the second oil channel, the oil groove 236, and the oil conduits 238, 240, 242, 244.
In addition to the phase adjustor, a lift adjustor employed in a variable valve timing apparatus to change the lift amount of intake or exhaust valves with a three-dimensional cam and to control the valve timing is also known in the prior art. Japanese Unexamined Patent Publication No. 9-32519 describes such a lift adjustor. As shown in FIG. 13, three-dimensional cams 302 are arranged on a camshaft 304. A timing pulley 306 is arranged on one end of the camshaft 304. The timing pulley 306 is supported such that it slides axially along and rotates integrally with the camshaft 304. A cylinder 308 is arranged on one side of the timing pulley 306. A piston 310 secured to the end of the camshaft 304 is fitted into the cylinder 308. A pressure chamber 312 is defined between one side of the piston 310 and the inner wall of the cylinder 308. A spring 314 is arranged between the other side of the piston 310 and the timing pulley 306 in a compressed state. When the pressure in the pressure chamber 312 is high, the piston 310 urges the camshaft 304 against the force of the spring 314 toward the right (as viewed in FIG. 13). When the pressure in the pressure chamber 312 is low, the spring 314 pushes the piston 310 and forces the camshaft 304 toward the left.
Hydraulic pressure is communicated between the pressure chamber 312 and an oil control valve 318 through oil conduits 322, 324, which extend through a bearing 320, oil conduits 326, 328, which extend through the camshaft 304, and an oil conduit 332, which extends through a bolt 330. The bolt 330 fastens the piston 310 to the camshaft 304. A microcomputer 316 controls the oil control valve 318 to adjust the hydraulic pressure communicated to the pressure chamber 312 and change the axial position of the camshaft if 304.
Accordingly, the position of contact between each cam 302 and the associated valve lift mechanism is adjusted to alter the opening duration of a corresponding intake valve or exhaust valve in accordance with the profile of the cam 302. This varies the valve timing.
When varying the valve timing with the phase adjustor illustrated in FIGS. 10 to 12, the opening and closing timing of the valves are both varied in the same manner. That is, if the opening timing is advanced, the closing timing is advanced accordingly, and if the opening timing is retarded, the closing timing is retarded accordingly. On the other hand, when varying the valve timing with the lift adjustor illustrated in FIG. 13, the opening and closing timing of the valves are inversely varied. That is, if the opening timing is retarded, the closing timing is advanced, and if the opening timing is advanced, the closing timing is retarded. Therefore, the opening and closing timing of the valves cannot be independently varied. This limits the control of the valve timing.
To solve this problem, the phase adjustor of FIGS. 10 to 12 and the lift adjustor of FIG. 13 can be arranged together on a camshaft to adjust both the rotational phase of a camshaft relative to a crankshaft and the lift amount of the valves. This would reduce the limitations on the opening and closing timing control.
For example, the phase adjustor of FIGS. 10 to 12 incorporating a timing pulley and a sprocket may be arranged on one end of a camshaft, and the lift adjustor of FIG. 13 may be arranged on the other end of the camshaft. In this case, the cylinder 308 of the apparatus illustrated in FIG. 13 is supported at a fixed position on a cylinder head or the like.
When employing the phase adjustor of FIGS. 10 to 12 together with the lift adjustor of FIG. 13, the phase adjustor must be unaffected by the camshaft axial movement that is caused by the lift adjustor of FIG. 13. A spline mechanism 406 such as that shown in FIG. 14 is thus required between a camshaft 402 and a vane rotor 404. The spline mechanism 406 includes splines 408, which extend along the inner surface of the vane rotor 404 and splines 414 extending along an inner gear 412, which is coupled to the camshaft 402. The vane rotor splines 408 and the inner gear splines 414 mesh with one another and are supported such that the gear splines 414 slide axially with respect to the vane rotor splines 408.
In this structure, the communication of hydraulic pressure may be performed in the conventional manner. For example, hydraulic pressure may be communicated from a bearing 416 to a first or second pressure chamber through an oil conduit 420, which extends through a sprocket 418 (the oil conduit 420 may extend through a timing pulley or gear instead), an oil conduit 422, which extends through the camshaft 402, an oil conduit 424, which extends through the inner gear 412, an interior space 426, which is defined in the vane rotor 404, and oil conduits 428, which connect the interior space 426 to the first or second pressure chamber.
However, the existence of the spline mechanism 406 causes difficulties when directly supplying hydraulic pressure from the oil conduit 424 of the inner gear 412, which is connected with the camshaft 402, to the oil conduits 428 of the vane rotor 404. More specifically, hydraulic oil must pass through the interior space 426 of the vane rotor 404 when sent to the oil conduits 428 of the vane rotor 404 from the oil conduit 424, which is connected with the camshaft 402.
Hydraulic oil passes through the interior space 426 regardless of whether the oil is sent to the first pressure chamber or second pressure chamber. Therefore, neither pressure chamber has an exclusive oil passage through which hydraulic oil is supplied. Furthermore, the hydraulic pressure communicated to the first and second pressure chambers cannot be controlled externally with the conventional structure. Accordingly, the vane rotor cannot be moved in a satisfactory manner unless a mechanism for independently supplying both of the pressure chambers with sufficient hydraulic pressure is provided or unless a spring such as that shown in FIG. 13 is used to exert force that substitutes for hydraulic force in one direction, while hydraulic force is applied in the opposite direction.
The interior space 426 would also cause a further problem. When varying the lift amount of the valves, the camshaft 402 moves axially relative to the vane rotor 404 and changes the volume of the interior space 426. Thus, the hydraulic pressure in the interior space 426 changes when the valve lifter varies the lift amount.
This may cause undesirable fluctuations of the pressure communicated through the oil conduits 420, 422, 424, 428, and the interior space 426. This would further interfere with the communication of sufficient hydraulic pressure to one of the pressure chambers.
Therefore, the installation of the phase adjustor of FIGS. 10 to 12 together with the lift adjustor of FIG. 13 on the same camshaft interferes with accurate control of the rotational phase of the camshaft relative to the crankshaft. This may lead to excessive retardation or excessive advancement of the valve timing, thus hindering accurate valve timing control.